This invention relates to phase-locked arrays of semiconductor lasers.
Phase-locked arrays of semiconductor lasers are increasingly finding application in optical recording, optical printing and free-space communications. A primary advantage over other light sources is their high power output while a secondary advantage is that required optical elements for focussing the output of a linear phased array are less complex than elements required for focussing a non-phased array laser output because in the former case the beam is emitted as a narrow lobe.
Known linear arrays of semiconductor lasers are typically formed as stripe contact devices on a bar of double heterostructure material derived from the gallium aluminum arsenide/gallium arsenide (GaAlAs/GaAs) or the gallium indium arsenide phosphide/indium phosphide (GaInAsP/InP) systems. The stripe contacts usually extend equispaced and parallel to one another along the length of a double heterostructure bar, the bar usually having common p and n side contacts. As stripe spacing is reduced below about 20 microns the characteristic random phase relationship between emissions from the lasers changes to one in which adjacent lasers are in anti-phase. In a few cases, in-phase operation does result, but this is fortuitous and unusual. The anti-phase operation is completely contrary to the desired relationship for higher brightness in which all emissions are in-phase.
Various attempts have been made to solve this problem. In one proposal individual contacts are made to each laser whereby the injection current is tailored to that offering the nearest approximation to phased emission. In another modification, in-phase emission has been achieved by adopting an irregular stripe spacing. An alternative solution is now proposed.